1. Field of the Invention
The present invention relates to network technology. More particularly, the present invention relates to methods and apparatus for implementing online restriping for distributed network based virtualization
2. Description of the Related Art
In recent years, the capacity of storage devices has not increased as fast as the demand for storage. Therefore a given server or other host must access multiple, physically distinct storage nodes (typically disks). In order to solve these storage limitations, the storage area network (SAN) was developed. Generally, a storage area network is a high-speed special-purpose network that interconnects different data storage devices and associated data hosts on behalf of a larger network of users. However, although a SAN enables a storage device to be configured for use by various network devices and/or entities within a network, data storage needs are often dynamic rather than static.
FIG. 1 illustrates an exemplary conventional storage area network. More specifically, within a storage area network 102, it is possible to couple a set of hosts (e.g., servers or workstations) 104, 106, 108 to a pool of storage devices (e.g., disks). In SCSI parlance, the hosts may be viewed as “initiators” and the storage devices may be viewed as “targets.” A storage pool may be implemented, for example, through a set of storage arrays or disk arrays 110, 112, 114. Each disk array 110, 112, 114 further corresponds to a set of disks. In this example, first disk array 110 corresponds to disks 116, 118, second disk array 112 corresponds to disk 120, and third disk array 114 corresponds to disks 122, 124. Rather than enabling all hosts 104-108 to access all disks 116-124, it is desirable to enable the dynamic and invisible allocation of storage (e.g., disks) to each of the hosts 104-108 via the disk arrays 110, 112, 114. In other words, physical memory (e.g., physical disks) may be allocated through the concept of virtual memory (e.g., virtual disks). This allows one to connect heterogeneous initiators to a distributed, heterogeneous set of targets (storage pool) in a manner enabling the dynamic and transparent allocation of storage.
The concept of virtual memory has traditionally been used to enable physical memory to be virtualized through the translation between physical addresses in physical memory and virtual addresses in virtual memory. Recently, the concept of “virtualization” has been implemented in storage area networks through various mechanisms. Virtualization interconverts physical storage and virtual storage on a storage network. The hosts (initiators) see virtual disks as targets. The virtual disks represent available physical storage in a defined but somewhat flexible manner. Virtualization provides hosts with a representation of available physical storage that is not constrained by certain physical arrangements/allocation of the storage. Some aspects of virtualization have recently been achieved through implementing the virtualization function in various locations within the storage area network. Three such locations have gained some level of acceptance: virtualization in the hosts (e.g., 104-108), virtualization in the disk arrays or storage arrays (e.g., 110-114), and virtualization in the network fabric (e.g., 102).
In some general ways, virtualization on a storage area network is similar to virtual memory on a typical computer system. Virtualization on a network, however, brings far greater complexity and far greater flexibility. The complexity arises directly from the fact that there are a number of separately interconnected network nodes. Virtualization must span these nodes. The nodes include hosts, storage subsystems, and switches (or comparable network traffic control devices such as routers). Often the hosts and/or storage subsystems are heterogeneous, being provided by different vendors. The vendors may employ distinctly different protocols (standard protocols or proprietary protocols). Thus, in many cases, virtualization provides the ability to connect heterogeneous initiators (e.g., hosts or servers) to a distributed, heterogeneous set of targets (storage subsystems), enabling the dynamic and transparent allocation of storage.
Examples of network specific virtualization operations include the following: RAID 0 through RAID 5, concatenation of memory from two or more distinct logical units of physical memory, sparing (auto-replacement of failed physical media), remote mirroring of physical memory, logging information (e.g., errors and/or statistics), load balancing among multiple physical memory systems, striping (e.g., RAID 0), security measures such as access control algorithms for accessing physical memory, resizing of virtual memory blocks, Logical Unit (LUN) mapping to allow arbitrary LUNs to serve as boot devices, backup of physical memory (point in time copying), and the like. These are merely examples of virtualization functions.
Some features of virtualization may be implemented using a Redundant Array of Independent Disks (RAID). Various RAID subtypes are generally known to one having ordinary skill in the art, and include, for example, RAID0, RAID1, RAID0+1, RAID5, etc. In RAID1, a virtual disk may correspond to two physical disks 116, 118 which both store the same data (or otherwise support recovery of the same data), thereby enabling redundancy to be supported within a storage area network. In RAID0, a single virtual disk is striped across multiple physical disks. Some other types of virtualization include concatenation, sparing, etc.
Generally, a striped configuration may be implemented in a storage volume made of n disks in order to distribute the data evenly across the disks in such a way as to maximize the number of drive spindles that are concurrently in use and thus maximize performance. The performance benefits may be realized for both read and write access.
Occasionally, it may be desirable to reconfigure the striping parameters of one or more disk arrays in a storage area network. Such reconfigurations may include, for example, changing the stripe unit size and/or changing the number of columns in the virtualized array. However, conventional techniques for implementing such reconfigurations typically require that the entire disk array be taken off line during the reconfiguration process. This results in service disruptions, which is undesirable. Additionally, conventional restriping techniques typically require the use of additional network resources such as, for example, the use of additional storage devices merely for implementing the restriping operations. For example, one technique commonly used for increasing the number of columns (e.g., from m columns to n columns) in a virtualized array is to copy the data from the existing array (of m columns) to a new array of physical disks which have been configured as a virtualized array of n columns. However, it can be seen that such a technique will require the use of m+n physical disks, even though the entirety of the data may be stored on n physical disks.
In view of the above, it would be desirable to improve upon restriping techniques implemented in virtualized disk arrays in order, for example, to provide for improved network reliability and efficient utilization of network resources.